1. Field of the Invention
The present invention relates to a filter which utilizes a surface acoustic wave (SAW), and more particularly to a SAW filter which is well suited to transmitters and receivers for mobile communications, especially for cellular radio. The SAW filter according to the present invention is applied to mobile communication equipments such as a pocket pager, mobile telephone and cordless telephone.
2. Description of the Related Art
A conventional SAW filter of the so-called transversal type has had a construction wherein an input interdigital transducer for converting an electric signal into a surface acoustic wave, and an output interdigital transducer for inverting the surface acoustic wave into an electric signal again are arranged on a piezoelectric substrate. An example of the SAW filter is stated in, e. g., Proceedings of IEEE, Vol. 67 (1979), pp. 129-146.
In the prior-art filter mentioned above, all the electric signals are once converted into surface acoustic wave signals by the input transducer, and the surface acoustic wave signals are inverted into the electric signals again by the output transducer. Therefore, loss attendant upon the processes of the conversion and the inversion is very heavy.
Moreover, when it is intended to input high electric power to such a filter, a surface acoustic wave of large amplitude is excited by the transducer due to the great input, so that migration (degradation or fatigue of metal) takes place in a very short time across the fingers of an interdigital electrode constituting the transducer of the filter (by way of example, in case of a filter for mobile telephone of 800 MHz band, the fingers of an aluminum electrode have a width of 1.1-1.2 .mu.m and a film thickness of 0.1 .mu.m). Since the progress of the migration leads even to the burnout or short-circuit of the electrode fingers, the input power to the filter needs to be suppressed below about 10 dBm (x[dBm]=10log{x[mW]1[mW]}) in the 800 MHz band by way of example. The technical problem of the migration is an important one to be solved and the solution is indispensable to enhancement in the lifetime and reliability of transmitter and receiver.
In addition, the advent of various communication means represented by portable telephone sets, as a consequence of the recent progress of communication technology, remarkably heightens a demand for communication wavelength bands to be used for them. The necessity for realizing many types of communication within a limited number of available wavelength bands requires improvement in the frequency characteristics of the transmitters and receivers.
The technical themes stated above are considered to be highly important problems which must be addressed in the field of future communication technology.